Everything depends on the sales. Anime covered 2 vol. of Light Novel out of 11 so they still got tons of material. Now they need $.
I read sales of the first DVD/BD are going well. They sold around 7.5k copies, usually this kind of shows reach 5-6k. Sekirei sold around 10k copies and another season was guaranteed, so yeah .... just a bit more to go
I was thinking that in the last minutes of the episode 12 Rias mentioned something about future engagements.
And the other thing is that I want to see the Angels fighting, since we saw the fallen angels, the bishops, and other demons in this season, but no Angels. Wishing it could be true.
Also:
I loled with the special. Rias surely knows how to get revenge
It's based on a light novel series so there's definitely more content, but it all depends how well the series did ratings wise if they will continue it. I'm still praying for continuations of kaze no stigma, Ore no Imouto ga Konna ni Kawaii Wake ga Nai and Boku wa Tomodachi ga Sukunai.
Then on the other hands you have series like Ikkitousen and Queen's blade which are nothing but boobs and panties and they each have like 3-5 seasons each. I love ecchiness as much as any other guy but there are so many great series out there that deserve additional seasons.
Although based on the above mentioned series that actually means that Highschool DxD does seem to have a good chance of getting another season since it does have plenty of the aforementioned requirements lol